Preparation of metal-coated powders and cathode structures



Oct. 1, 1968 D w MAURER ET AL 3,404,034

PREPARATIONS OF METAL COATED POWDERS AND CATHODE STRUCTURES 3Sheets-Sheet 1 Filed Nov. 15, 1967 FIG. 2

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#snmhmrlvqm zitiiaizizfl m I n 0. W MAURER INVENTOPS CM. PLEASS ATTORNEYOct. 1, 1968 D. w. MAURER ET 3,404,034

PREPARATIONS OF METAL COATED POWDERS AND CATHODE STRUCTURES Filed Nov.15, 1967 5 Sheets-Sheet 2 FIG. 3

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3 200 g EST/MA r50 smcg CHARGE L/M/T q FOR THIS CONFIGURATION S |5o 3Q'mo NORMAL SPACE CHARGE L/M/T roe NICKEL MATR/X 50 A7 750 5 O l I l l olOO 200 300 400 500 VOLTAGE Oct. 1, 1968 D. w. MAURER E AL 3,404,034

PREPARATIONS OF METAL COATED POWDERS AND CATHODE STRUCTURES Filed Nov.15, 1967 5 Sheets-Sheet 5 FIG. 5

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(SPACE CHARGE L/M/T FOR THIS CONF/GURAT/ON 4oo I N 9 300 Q: u Q E 3 \1200 t E SPACE CHARGE L /M/T FOR N CONVENTIONAL MATRIX CATHODE lOO O I ll l O 50 IOO I50 200 VOLTAGE United States Patent 3,404,034 PREPARATIONOF METAL-COATED POWDERS AND CATHODE STRUCTURES Dean W. Maurer, BerkeleyHeights, and Charles M.

Pleass, Reiifton, N.J., assignors to Bell Telephone Labgfratories, Inc.,Murray Hill, N.J., a corporation of New ork Continuation-impart ofapplication Ser. No. 520,488, Jan. 13, 1966, which is acontinuation-in-part of application Ser. No. 310,040, Sept. 19, 1963.This application Nov. 15, 1967, Ser. No. 683,378

6 Claims. (Cl. 117224) ABSTRACT OF THE DISCLOSURE Coated and matrix typecathode element destined for use in thermionic tubes include a basemember bearing a coating of an emissive material in particulate form,the particles of which have been previously coated with a thin film of ametal.

This application is a continuation-in-part of copending application Ser.No. 520,488, filed Ian. 13, 1966, which is in turn acontinuation-in-part of copending application Ser. No, 310,040, nowabandoned, filed Sept. 19, 1963.

This invention relates to a technique for coating discrete particulatematerial and, more particularly, to a cathode structure including metalcoated thermionically active powders which may be coated thereby.

There are three fundamental types of cathode structure in commercial useat this time. The earliest and most conventional type comprises a solidbase having a coating of an alkaline earth metal oxide generallyincluding barium oxide. The second type comprises a porous pressuremolded tungsten matrix which is impregnated with barium aluminate. Thethird type of cathode structure in existence today is the nickel matrixcathode which includes a molded element made from a pressed and firedmixture, generally including nickel powder together with an alkalineearth metal oxide.

In general, each of these three types of cathode structures has certainadvantages and disadvantages which dictate selection for a particularuse. Thus, for a given configuration and operation conditions, the oxidecoated cathode is capable of delivering a considerably higher currentdensity than either matrix type structure. On the other hand, the matrixcathodes contain a reservoir of active material which is utilized tocontinually replenish the active emitting surface layer during life.Accordingly, matrix structures are considered more desirable for useunder more adverse conditions, as for example, where there is a highdegree of back bombardment, or under other conditions which may causedeterioration of the relatively thin oxide coating of the moreconventional structure such as sustained direct-current emission greaterthan approximately 0.4 ampere per square centimeter.

In accordance with the present invention, a technique is described forthe fabrication of both coated and matrix type cathodes including metalcoated thermionically active powders. The inventive technique involvescoating discrete particles of such powders with a thin film of a metalcapable of forming a thermally unstable compound, coating being effectedby conventional dry fluidization or plating techniques or by means of anovel wet fluidization technique. Claims in the application are directedto the novel fluidization technique and to the cathode structure andmethod for the preparation thereof. The particles so coated are thenemployed as the thermionically active materials in numerous cathodestructures, so resulting in a group of devices manifesting highercurrent densities at 3,404,034 Patented Oct. 1, 1968 lower operatingtemperatures than have heretofore been attained by any prior art cathodestructure.

The invention will be more easily understood from the following detaileddescription taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a schematic diagram of dry fluidized bed system used in thepractice of the present invention;

FIG. 2 is a schematic diagram of a novel, typical, wet fluidized systemused in the practice of the present invention;

FIG. 3 is a cross-sectional view of a cathode structure fabricated inaccordance with the present invention;

FIG. 3A is a cross-sectional view of metal coated thermionically activeparticles prepared as described;

FIG. 4 is a graphical representation on coordinates of current inmilliamperes to the two-thirds power against voltage in volts showingthe space charge break for a plasma sprayed cathode of the presentinvention at 750 B. after 720 hours of life; and

FIG. 5 is a graphical representation on coordinates of current inmilliamperes to the two-thirds power against voltage in volts showingthe space charge break for an air sprayed cathode of the presentinvention at 750 B. after 315 hours of life.

A general outline of the procedure employed in fabricating the novelstructures described herein together with the ranges of operatingparameters will now be given.

The first step of the inventive technique involves coating discreteparticles of a thermionically active material with a thin film of ametal. Typically, the particulate material is an alkaline earth oxide,or carbonate depending upon the particular configuration desired. Thesematerials are conventional emitting materials and commonly employed inthe preparation of sprayed oxide and matrix cathodes.

Metals found suitable for coating in accordance with the presentinvention may be selected from among those metals which are compatiblewith the functioning of the cathode and are capable of forming thermallyunstable compounds over a practical temperature range. Metals foundparticularly suitable in this use are tungsten, molybdenum, nickel andcobalt. I

Coating of the discrete particulate material may be effected by anyconventional coating or plating technique, as for example, dryfluidization, barrel plating et cetera. Additionally, coating may beeffected by a novel wet fluidization technique. It will be appreciatedby those skilled in the art that one objective of the wet fluidizationmethod described herein is to avoid agglomeration of discreteparticulate material during the coating operation, such being a majorprior art problem. As employed herein, this novel technique isspecifically directed to the coating of thermionically active particles,destined for use in cathode elements, with a thin film of tungsten,molybdenum, nickel or cobalt. However, it is evident that this techniqueis not restricted to the noted metals or even to metals and may beemployed in any operation resulting in deposition of a material by meansof thermal decomposition.

With further reference now to FIG. 1, there is shown a schematic diagramof a dry fluidized bed system which may be employed in the practice ofthe invention as one means for coating the active materials. Shown inthe figure is stainless steel fluidization column 11 which is connectedto glass column 12 by means of polyethylene joint 13. At the lowerextremity of column 11 there is shown a porous stainless steel sinteredfrit 14 which completely obscures the diameter thereof, frit 14 beingbrazed into column 11. Glass frit 15 of the same porosity as frit 14 issimilarly shown fused into column 12 at the upper extremity thereof toprevent the loss of powder in the stream of fiuidizing gas duringoperation. Column 11 is heated by means of heating coils 16, therebyproviding the requisite heat for decomposition of the metal compoundduring the coating process. Shown connected to column 12 by means ofconduit 17 is drying tube 18 through. which the gaseous products of theprocess pass prior to being ignited at the exit end of the system. Shownconnected to column 11 by means of conduit 19 is bubbler 20 whichcontains a metal compound 21 capable of decomposing thermally during theoperation of the process. The system is completed by flowmeter 22positioned at the entrance end of the system through which fluidizinggas enters from a source not shown. Bypass conduit 23 and valves 24, and26 are employed for controlling the process.

In the operation of the process, a suitable (non-oxidizing) fiuidizinggas, for example, hydrogen, nitrogen or argon, depending upon theparticles being coated, is admitted to the system at the entrance end,passes through flowmeter 22 and with valves 24 and 25 in the closedposition and valve 26 in the open position, passes through bypassconduit 23 and conduit 19 into column 11 which is heated by means ofheating coils 16 for a suitable period of time required to effect abake-out of the system. Next, the thermionically active material whichhas previously been ball milled to the required particle size, generallywithin the range of 1-5 microns, is introduced into the system andfiuidization initiated, the gas being employed therefor being hydrogenor any of the gases described above. Following, the effluent is ignitedat the exit end of the system and burning continued throughout theprocess. Next, hydrogen is diverted from bypass conduit 23 by closingvalve 26 and opening valves 24 and 25, thereby permitting the gas topass through bubbler 20 and thence to column 11 wherein the metalcompound 21 decomposes at elevated temperatures to yield an elementalmetal which coats the thermionically active particles. The coatedparticles are subsequently removed from the system and stored untilready for use in the fabrication of a cathode element.

In an alternative inventive technique for coating the discreteparticulate material, the apparatus shown in FIG. 2 is employed. Thissystem has conveniently been termed wet fiuidization. In this system,columns 11 and 12 (of FIG. 1) are replaced by fluidization column 30containing an inert fluid 31 and having a suspension of finely dividedthermionically active materials of the type described above. Column 30is heated by means of a constant temperature bath 32. A suitablestirring device 33, typically a magnetic stirrer, assures the requisiteagitation of the particulate material during coating. Once again,bubbler 20 contains a metal compound 21 capable of decomposing thermallyduring the operation of the process. It will be understood that themetal compound utilized in the practice of the present invention may beany organo metallic compound capable of decomposing thermally during theoperation of the described process, and it may be either a liquid or asolid. However, it has been found convenient to employ carbonyl ordicyclopentadienyl compounds of the metals of interest. Nickel carbonyl,molybdenum carbonyl, and dicyclopentadienylnickel have been found to beparticularly advantageous in the practice of the invention, a generalpreference being shown for the dicyclopentadienylnickel compound. Itwill also be understood that when the dicyclopentadienyl compounds areemployed they may advantageously be dissolved in the same liquid thatsuspends the particles to be coated and introduced directly to thereaction vessel.

In order that those skilled in the art may more fully understand theinventive concept herein presented, the following examples are given byway of illustration and not limitation.

EXAMPLE I This example describes the fabrication of a cathode structurewherein nickel coated alkaline earth oxides (barium-strontiumcoprecipitated) are plasma sprayed upon a solid active alloy base.

Coprecipitated barium-strontium peroxide was placed in a boatconstructed of Driver Harris No. 499 nickel, a high purity passivematerial. Next, the boat was inserted in a quartz tube furnacemaintained under vacuum and heated at 900 C. for a time period of hours,thereby causing decomposition of the peroxides to the correspondingoxides in accordance with Equation (1). The pressure at the conclusionof heating was approximately 10- torr.

2(BaS1')Oz 2(BaSr)0 on The coarse product was then transferred to aPyrex mill jar containing aluminum oxide balls and ball milled for 36hours, thereby forming a fine barium-strontium oxide powder havingmaximum particle size of 37. The resultant fine powder was then chargedto a precleaned and prebaked fiuidization column of the type illustratedin FIG. 1. Fluidization was initiated by admitting a stream of hydrogensaturated with carbonyl derived nickel from bubbler 20 at roomtemperature and coating attained by heating the fluidization column to atemperature of C. for 20 hours, thereby causing decomposition of thecarbonyl and concomitant coating of the barium-strontium oxide particleswith a film of nickel. The coated particles contained 14 percent byweight nickel and 86 percent by weight barium-strontium oxide.

Two cathode buttons (machine plugs) of 0.1 percent zirconium-nickelalloy, having a diameter of 0.085 inch were selected and the topsurfaces thereof grit blasted with aluminum oxide grit and subjected toa conventional cleaning procedure for oxide cathode bases. The cleaningtechnique involved racking the caps in a nickel-zirconium boat andsubjecting the caps to a conventional vapor degreasing technique. Next,the caps were blown dry with low pressure nitrogen and ultrasonicallywashed. Following, the washed caps were rinsed in cascading deionizedwater, dried in an air oven at C. for 15 minutes, oxidized in air at 400C. for 20 minutes and reduced in wet hydrogen at 1050 C. for 30 minutes.Following, the cleansed buttons were mounted in a jig and plasma-spraycoated with the nickel coated barium-strontium oxide particles to athickness of 3 mils in accordance with the following procedure.

The coated particles were deposited by means of a direct-current arcplasma gun wherein hydrogen was ionized by passage through a high powerdirect-current arc, thereby forming a highly energetic plasma downstreamfrom the are at which point the recombination energies of the ionicspecies produced was translated into thermal energy of the gas atoms.The introduction of the discrete particulate material into this highenergy area renders them molten. The molten particles were thenpermitted to impinge upon a substrate, the cathode buttons, where theycoalesced to form a dense coating.

One of the buttons so obtained was then fired for 15 minutes at 800 C.in a hydrogen ambient in a conventional furnace and subsequently coinedunder an applied pressure of 50 tons per square inch. Then, the buttonwas placed in a molybdenum heater sleeve and sintered by firing for 15minutes at 1000 C. in a hydrogen ambient.

The other cathode button was initially placed in a molybdenum heatersleeve and sintered as described above.

FIG. 3 is a cross-sectional view of a cathode element prepared inaccordance with the technique described above. Shown in the figure as abase region 41 including nickel together with an activator and a coating42 comprising metal coated thermionically active particles 43, theparticles 43 being shown in greater detail in FIG. 3A.

When ready for use, the cathode elements so produced were assembled in atube envelope by conventional techniques and sealed to a vacuum systemin which a vacuum of millimeters of mercury could be attained, and inwhich the structure was baked for 16 hours at 400 C. After bake-out,cathode heater voltage was applied to increase the cathode temperatureto 1050 C. at which it was maintained for 5 minutes. Next voltage wasapplied to the anode until a cathode current of 1 amp/cm. was attained.The tube was then sealed off the station. The completed diode was thenplaced on a life test rack and its operating characteristics observed.

The full impact of the present invention can best be seen by referenceto FIG. 4. The data reflected therein was obtained by placing thecathode elements prepared as described in Example I on a life test rackand applying 200 volts to the anodes. After 720 hours of life the directcurrent of each was measured as a function of the anode voltage at 750B. The data obtained was then plotted on a graph having current inmilliamperes to the twothirds power as one coordinate and voltage as theother coordinate.

It is noted that the space charge limited emission of the two cathodesfabricated in accordance with the inventive technique is approximatelymilliamperes at 750 B. (same curve for each) as compared with a maximumspace charge limited emission of 10 milliamperes for conventional nickelmatrix cathodes, a significant advance from the standpoint of cathodetechnology.

EXAMPLE II This example describes the fabrication of a cathode structurewherein nickel coated alkaline earth carbonates (barium-strontium) areair sprayed upon a solid active alloy base.

80 g. SrCO 72 g. BaCO and 200 cc. of amyl acetate were placed in a milljar containing flint stones and ball milled for 64 hours, so forming afine suspension of carbonate powder in amyl acetate. The resultantsuspension was then charged to a fluidization column (30) of the typeillustrated in FIG. 2, the fiuidization column being immersed inconstant temperature oil bath 33. Fluidization was initiated byadmitting a stream of hydrogen containing nickel carbonyl vapor into thefluidization column and coating attained by heating the column by meansof oil bath 33 toa temperature within the range of 80-90 C. for 22hours, thereby causing decomposition of the carbonyl and coating of thecarbonates with a thin film of nickel.

The apparatus was next dismantled and the carbonates separated from theamyl acetate by filtration and drved in air at 110 C.

100 g. of the coated carbonates were then mixed with 75 ml. of amylacetate and 82 cc. of a nitrocellulose binder solution in order to forma carbonate mix.

A cathode button of 0.1 percent zirconium-nickel alloy, having adiameter of 0.085 inch was selected and cleaned in accordance with theprocedure described in Example I. The carbonate mix was then sprayedupon the cathode with a conventional artists air brush, a coating of 0.5mil in thickness being formed. Following, the sprayed cathode was firedat 250 C. in oxygen to burn oif the binder.

When ready for use, the cathode so produced was as sembled in a tubeenvelope by conventional techniques and sealed to a vacuum system inwhich a vacuum of 10" millimeters of mercury could be attained and inwhich the structure was baked overnight at 400 C. After bake-out,cathode heater voltage was applied to increase the temperature to 800 B.for outgassing at which it was maintained for 100 minutes or until thepressure was 4x10 torr, Next, the temperature was increased to 950 B.and maintained thereat for 5 minutes. Following, the temperature wasdecreased to 850 B. and voltage ap plied to the anode until a cathodecurrent of 0.5 amp/cm. was attained. The tube was then sealed off thestation, placed on a life test rack, and aged.

The data reflected in FIG. 5 was obtained by placing the cathodeprepared as described in Example II on a life test rack and applyingvolts to the anode. After 315 hours of life, the direct current wasmeasured as a function of the anode voltage at 750 B. The data obtainedwas then plotted on a graph having current in milliamperes to thetwo-thirds power as one coordinate and voltage as the other coordinate.

It is noted that the space charge limited emission of the cathodefabricated in accordance with the inventive technique is approximately48 milliamperes at 750 B. as compared with a maximum space chargelimited emission of 10 milliamperes for conventional matrix cathodes.

EXAMPLE III The procedure of Example I was repeated with the exceptionthat fluidization was initiated by admitting a stream of hydrogensaturated with dicyclopentadienyl derived nickel from bubbler 20 at roomtemperature. The characteristics of the resultant cathode button werefound to be identical to those of the structure described in Example I.

While the invention has been described in detail in the foregoingspecification and the drawing similarly illustrates the same, theaforesaid is by way of illustration only and is not restrictive incharacter. The several modifications which will readily suggestthemselves to persons skilled in the art all considered within the scopeof this invention, reference being had to the appended claims.

What is claimed is:

1. A method for the fabrication of a cathode element destined for use ina thermionic tube comprising the steps of (a) coating discreteparticulate material selected from the group consisting of (1) at leastone alkaline earth oxide and (2) at least one alkaline earth carbonatewith a metal film selected from the group consisting of nickel,tungsten, molybdenum, and cobalt by passing a non-oxidizing gas selectedfrom the group consisting of hydrogen, nitrogen, and argon over acompound selected from the group consisting of carbonyl anddicyclopentadienyl compounds of said metal which are capable ofdecomposing thermally to yield the said metal and passing the eifiuenttherefrom into an inert liquid capable of reacting therewith and whichcomprises a suspension of said discrete particulate material, the saidliquid being at a temperature suflicient to decompose said metalcompound, thereby resulting in the coating of said particulate materialand (b) depositing a coating of the resultant coated particulatematerial upon a solid base member comprising nickel.

2. A method for coating discrete particulate material with a thin filmof at least one metal selected from the group consisting of nickel,tungsten, molybdenum, and cobalt, comprising the steps of passing anon-oxidizing gas selected from the group consisting of hydrogen,nitrogen, and argon over a compound selected from the group consistingof carbonyl and dicyclopentadienyl compounds of said metal, the saidcompound being capable of decomposing thermally and passing the efiluenttherefrom into an inert liquid incapable of reacting therewith, the saidliquid comprising a suspension of said discrete particulate materialheated to a temperature suflicient to decompose said compound, therebyresulting in the coating of said particulate material.

3. A method in accordance with the procedure of claim 2 wherein saiddiscrete particulate material is an alkaline earth oxide and said metalcompound is nickel carbonyl.

4. A method in accordance with the procedure of claim 2 wherein saiddiscrete particulate material is an alkline earth carbonate and saidmetal compound is nickel carbonyl.

5. A method in accordance with the procedure of claim 2 wherein saidnon-oxidizing gas is hydrogen, and said suspension comprises discreteparticulate material suspended in amyl acetate.

6. A method in accordance with the procedure of claim 7 2 wherein saiddiscrete particulate material is an alkaline earth oxide and said metalcompound is dicyclopentadienylnickei.

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